The present invention relates to household mixing-valve devices and, more particularly, to household electronic thermostatic mixing-valve devices.
In the household, electronic thermostatic mixing valves or faucets may be used to mix relatively hot and cold water streams to provide a mixed stream of a substantially fixed temperature, by electronically adjusting one or more valve members in response to a set point, typically a set point of the user. Such mixing valves may be installed in the bathroom or shower, by way of example.
Though the design of such a device might appear to be fairly straightforward, there exist numerous and often-contradictory requirements for the design, including response time, safety, reliability, and ease of maintenance. Of course, the design is also constrained by the need to produce and provide the device at a cost that enables market penetration.
Electronic faucets typically implement a closed-loop control of some kind, which may potentially become unstable, and may result in the dispensing of dangerously-hot water to the user. It is therefore highly important to eliminate, minimize, or at least greatly reduce the possibility of such instabilities.
In domestic water systems, the task of stable control over the mixed water stream parameters is complicated by the diversity of the inlet conditions to the mixing valve. For example, the cold water temperature may vary from virtually 0° C. in cold weather to as much as 30° C. in hot weather. The hot water temperature may be as high as 80° C. or more, e.g., when the hot water is drawn directly from a solar boiler or gas heater, and may be as low as the temperature of the cold water. Typically, the hot water temperature may lie within a rather broad range of 40° C. to 75° C.
Significantly, the inlet pressures to the mixing valves may vary, or fluctuate, within a range of about 1.5 to 7 bar (gauge) depending on the supplier, the consumption, and the height of the consumer location.
Also significantly, the outlet may be supplied with a flow restricting device, limiting the flow rate through the device. The use of restriction is required by some of the modern codes. While approaching the limitation flow rate, most of the inlet pressure falls on the outlet restriction while a small portion of the pressure is applied to the mixing valve itself. The resulting low flow rate also reduces the response time of temperature sensing elements that may be installed inside the mixing valve, due to reduced convection.
Household thermostatic mixing faucets may require sub-second response times, in order to effectively respond to abrupt situations when the cold supply pressure momentarily drops, for example, after an abrupt opening of a connected, alternative or auxiliary water conduit, or due to a catastrophic failure or explosion of a cold-water pipe.
In relating to the control of thermostatic mixing faucets, some known devices utilize a single temperature sensor on the mixed flow to provide a feedback for the control loop.
Other known devices are disclosed by U.S. Patent Publication No. 20060231638 and U.S. Pat. No. 4,756,030, both of which are incorporated by reference for all purposes as if fully set forth herein. The flow through the two inlets is measured together with the inlet temperatures, and an additional temperature sensor may be added to measure the temperature of the mixed flow. Based on the set points, the measured inlet temperatures, and using Richmann's rule of mixing, the required flow rates through each inlet are calculated. A controller uses the measurements from the flow sensors and moves the valves in order to maintain the calculated flow.
Other known devices are disclosed by U.S. Pat. No. 4,694,512.
While various electronic thermostatic mixing faucets are known, to date, penetration into the household market has been limited. And while some technological advances have been made, the present inventor has recognized a need for further improvements in the response behavior, safety, robustness, and ease of maintenance, while provided a cost-effective design and product. The subject matter of the present disclosure and claims is aimed at fulfilling this need.